Ancient Crocodile Fossil Defies Dinosaur Label

July 6, 2005

In the world of dinosaurs, the species known as Revueltosaurus callenderi was the Rodney Dangerfield of dinosaurs. It was only about three or four feet long, an ugly critter that is little more than a footnote in the dinosaur record.

But these days it's getting a lot more respect. Until last year, paleontologists had no more than teeth and a few scattered bones, which may or may not have come from the same animal, but they were pretty sure that Revueltosaurus was a member of one of two lines of dinosaurs that emerged at about the same time, during the late Triassic Period more than 210 million years ago.

Since teeth are usually the only thing that survives in the fossil record for that many years, numerous other animals with teeth similar to Revueltosaurus's were classified as dinosaurs also.

And that's the way it stayed until Bill Parker, a paleontologist at the Petrified Forest National Park in Arizona, dug down in the dry soil there last year and found a complete skull -- and later an entire skeleton -- of Revueltosaurus, teeth and all.

That little find has sent the world of paleontology into a tailspin. Revueltosaurus, it turns out, wasn't a dinosaur at all. It was an early ancestor of a modern crocodile.

The Triassic's Two Classes of Dinos

It's a stunning development because Revueltosaurus is only one of many late Triassic animals known only by their teeth that were thought to be the ancestors of plant-eating dinosaurs. But the fact that the teeth came from an early crocodile, and not from a dinosaur, suggests that many of the animals that roamed North America more than 200 million years ago have been erroneously classified as dinosaurs.

"Because the teeth look like those we know from herbivorous ornithischians (one line of dinosaurs) people assigned them to the dinosaurs," says Randall Irmis, a graduate student at the University of California, Berkeley, who has been working with Parker. But the latest discovery shows that the dental record is not a reliable factor, and that "casts doubt on all the ornithischians from the Triassic of North America," Irmis says.

"There's two main groups of dinosaurs, ornithischian dinosaurs like Stegosaurus and Triceratops, and then there's the saurischian dinosaurs which are your big dinosaurs like Brontosaurus and later, large meat-eating forms," Parker says.

"The Triassic is supposedly the period when both of these branches formed, and we have lots of saurischian dinosaurs, but the ornithischian record is really, really spotty. All we have are teeth which are supposed to be ornithischian. So by finding a crocodilian with these teeth suddenly you can't say that all these teeth belonged to dinosaurs. We've wiped away the entire ornithischian dinosaur record for the late Triassic," Parker says.

Parker and his colleagues reported their findings in the journal of the Proceedings of the Royal Society, Biological Sciences.

The Old Stomping Grounds

The animal they exhumed once roamed an area that looked far different than it does today. The Petrified Forest National Park, known primarily for the fossilized remains of what was once a great forest, is also one of the richest repositories of ancient fossils in North America, if not the world.

Today it is an arid desert, but in the late Triassic it was more like an oasis with rivers that harbored all sorts of life.

Parker wasn't even looking for fossils when he was roaming through part of the park, known as the Painted Desert, with several geologists. They were looking for clues to the geological history of the region.

"I saw this very promising outcrop," Parker says, which he immediately recognized as an ancient flood plain.

"That's the best place to find this stuff," he says of the fossils that now threaten to consume his life.

"This was a huge river system," he says. "During the Triassic a lot of things died along the river channels. And during periods of heavy rain the rivers would flood and there would be a lot of sediments, so there was a quick burial. That's pretty much the key to preserving fossils."

Uncovering the Tiny Croc

Parker returned to the same site the next day with a colleague, and "sure enough, within minutes after I got there, I found all these armor plates, crocodilian type plates" that helped protect some early animal from predators, he says.

Minutes later they found a piece of a jaw.

"This was the first time anyone had actually picked up jaw material, or any material other than just isolated teeth that were laying on the ground," he adds.

They kept digging and soon hit the jackpot.

"There was a skull there, and it had Revueltosaurus teeth in it," he says. "So light bulbs went off and I realized that Revueltosaurus was not an ornithischian dinosaur. Instead, it was this little armored, crocodile ancestor."

More digging revealed more evidence. The animal had an ankle like a crocodile, not a dinosaur.

"Dinosaurs have kind of a bird-like ankle that allows for a more upright posture, whereas the crocodile ankle is weaker, and it causes the animal to be sprawling," Parker says.

Paleontological Puzzle

As is so often the case, the discovery has precipitated more questions than answers because it clouds the origin of an entire line of dinosaurs.

"Now we don't know where the ornithischian came from," Parker says. "Where do they fit in?"

It was thought they may have originated in South America, then extended their range across continents that have long since separated. But the evidence for early ornithischian in South America is also largely dental, so who knows?

The researchers say their discovery makes it clear, however, that ornithischian diversified much later than saurischian, and many animals thought to be dinosaurs probably aren't.

Not a bad contribution from a strange little animal that probably ended up on the food chain for an expanding population of meat-eating dinosaurs.

Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.